Due to the wide-spread use of network techniques such as the Internet and an intranet, distribution and sharing of various types of data, such as documents, video and still image content, and engineering data, over a network are quite common. Since some of the data have confidential information or are copyrighted, there are used techniques for protecting such data and for investigating the leaker of confidential information or the copyright infringer. One technique is to scramble distributed data by embedding noise therein, and another is to embed data used for tracking (generally called “forensic data”).